High quality hole cutting using variable shield gas compositions

ABSTRACT

A method and apparatus for a plasma torch system having a plasma torch tip configuration that includes a nozzle, an electrode, and a control unit for controlling a composition of the shield gas flow, such that while cutting the contour the shield gas flow comprises a first shield gas composition and while cutting the hole the shield gas flow comprises a second shield gas composition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of and claims the benefitof U.S. patent application Ser. No. 12/341,731, file on Dec. 22, 2008,the content of which is incorporated herein by reference in itsentirety.

TECHNICAL FIELD

The invention relates generally to plasma arc cutting torches. Morespecifically, the invention relates to a method and apparatus forcutting holes and contours in a workpiece using a plasma torch tipconfiguration.

BACKGROUND

Plasma cutting is commonly carried out by using a constricted electricarc to heat a gas flow to the plasma state. The energy from the hightemperature plasma flow locally melts the workpiece. For many cuttingprocesses, a secondary gas flow (also known as a shield gas flow, orshield flow) is used to protect the torch and assist the cuttingprocess. The momentum of the high temperature plasma flow and the shieldflow help remove the molten material, leaving a channel in the workpieceknown as a cut kerf.

Relative motion between the plasma torch and the workpiece allows theprocess to be used to effectively cut the workpiece. The shield gasinteracts with the plasma gas and the surface of the workpiece and playsa critical role in the cutting process. Downstream of the nozzleorifice, the plasma and shield gas flows come into contact enabling heatand mass transfer.

For reference, FIG. 1 is a diagram of a known automated plasma torchsystem. Automated torch system 10 can include a cutting table 22 andtorch 24. An example of a torch that can be used in an automated systemis the HPR260 auto gas system, manufactured by Hypertherm®, Inc., ofHanover, N.H. The torch height controller 18 is then mounted to a gantry26. The automated system 10 can also include a drive system 20. Thetorch is powered by a power supply 14. An automated torch system 10 canalso include a computer numeric controller 12 (CNC), for example, aHypertherm Automation Voyager, manufactured by Hypertherm®, Inc.,Hanover, N.H. The CNC 12 can include a display screen 13 which is usedby the torch operator to input or read information that the CNC 12 usesto determine operating parameters. In some embodiments, operatingparameters can include cut speed, torch height, and plasma and shieldgas composition. The display screen 13 can also be used by the operatorto manually input operating parameters. A torch 24 can also include atorch body (not shown) and torch consumables that are mounted to thefront end of a torch body. Further discussion of CNC 12 configurationcan be found in U.S. Patent Publication No. 2006/0108333, assigned toHypertherm®, Inc., the entirety which is incorporated herein byreference.

FIG. 2 is a cross-sectional view of a known plasma arc torch tipconfiguration, including consumable parts and gas flows. The electrode27, nozzle 28, and shield 29 are nested together such that the plasmagas 30 flows between the exterior of the electrode and the interiorsurface of the nozzle. A plasma chamber 32 is defined between theelectrode 27 and nozzle 28. A plasma arc 31 is foamed in the plasmachamber 32. The plasma arc 31 exits the torch tip through a plasma arcorifice 33 in the front end of the nozzle. The shield gas 34 flowsbetween the exterior surface of the nozzle and the interior surface ofthe shield. The shield gas 34 exits the torch tip through the shieldexit orifice 35 in the front end of the shield, and can be configured tosurround the plasma arc. In some instances, the shield gas also exitsthe torch tip through bleed holes 36 disposed within the shield 29. Anexample of plasma torch consumables are the consumable partsmanufactured by Hypertherm®, Inc., of Hanover, N.H. for HPR 130 systems,for cutting mild steel with a current of 80 amps.

A portion of the shield gas flow can enter the cut kerf with the plasmagas and form a boundary layer between the cutting arc and the workpiecesurface 37. The composition of this boundary layer influences the heattransfer from the arc to the workpiece surface and the chemicalreactions that occur at the workpiece surface.

Numerous gas mixtures are used for both plasma and shield gas in plasmacutting processes. For example, oxygen is used as the plasma gas and airas the shield gas for the processing of mild steel. Some low currentprocesses (e.g., less than 65 A) use oxygen as both the plasma gas andshield gas to cut thin material (e.g., workpieces less than 10 gauge).The oxygen plasma gas/air shield gas combination is popular for mildsteel at arc currents above 50 amps, due to the ability to produce largeparts with good quality and minimal dross at high cutting speeds. Suchcutting processes have certain drawbacks. For example, though the oxygenplasma gas/air shield gas configuration can cleanly cut large sectionswith straight edges, such a gas combination is unable to create highquality holes. Instead, holes cut with oxygen plasma gas and air shieldgas has a substantial “bevel” or “taper.” Bevel or taper occurs wherethe diameter at the bottom side of the workpiece is smaller than thediameter at the top side of the plate. In a bolt hole cut using an airshield gas, if the diameter of the hole at the top of the workpiece iscut to match the size of the bolt which is to pass through the hole, thetaper of the hole cut with an air shield gas may cause the hole diameterat the bottom of the workpiece to be smaller than the diameter of thebolt, preventing the bolt from passing through the bottom of theworkpiece. In these types of instances, secondary processes, such arereaming or drilling is required to enlarge the diameter of the bolt holeat the bottom of the workpiece. This prior method of ensuring hole cutquality was time consuming suggesting that a more efficient method ofcutting holes and contours in a single workpiece is necessary.

SUMMARY OF THE INVENTION

The present invention substantially improves the cut quality for smallinternal part features, or holes, while maintaining the productivity andcut quality for large features, or contours. By changing the shield gascomposition when cutting a hole and a contour in a single workpiece, thepresent invention eliminates the need for secondary processes, Forexample, while cutting the contour the shield gas flow can have a firstshield gas composition and while cutting the hole the shield gas flowhas a second shield gas composition.

In one aspect, the invention features a method for cutting a hole and acontour in a workpiece with a plasma torch. In one embodiment the methodincludes a plasma torch including a nozzle and electrode that define aplasma chamber; a plasma arc is generated in the plasma chamber. In oneembodiment, the plasma torch also includes a shield gas supply line forproviding a shield gas flow to the plasma arc torch, and a control unitfor controlling cutting parameters including cutting speed and shieldgas composition. In one embodiment the method includes controlling thecutting parameters such that when the contour is cut the shield gascomprises a first shield gas composition and when the hole is cut theshield gas comprises a second shield gas composition. In someembodiments, the first shield gas composition is different than thesecond shield gas composition.

In still another aspect the invention features a method for improvingthe cutting characteristics of a small internal feature in a plasmatorch cutting operation. In one embodiment the method includes the stepsof cutting a small internal feature using a second shield gascomposition, the small internal feature positioned within theanticipated contour cut of a workpiece, and cutting a contourcorresponding to the anticipated contour cut using a first shield gascomposition.

In a further aspect the invention features a method for cutting a holeand a contour in a workpiece using a plasma arc torch. The plasma arctorch can include a nozzle and electrode that define a plasma chamber,such that a plasma arc generated in the plasma chamber is used to cutthe workpiece, and a shield gas supply line that delivers a shield gasflow to the plasma torch. In one embodiment, the method includes thestep of cutting a hole in a workpiece wherein the shield gas flowcomprises a second shield gas composition which is selected such that abevel of an edge of the hole is substantially eliminated. In oneembodiment the method can also include the steps of cutting a contourwherein the shield gas flow comprises a first shield gas composition,and controlling the first shield gas composition and the second shieldgas composition such that while cutting the hole the second shield gascomposition comprises less nitrogen than the first shield gascomposition.

In another aspect, the invention features a further method of cutting ahole in a workpiece using a plasma arc torch. The method can include aplasma arc torch including high-current consumables, the high-currentconsumables including a nozzle and electrode that define a plasmachamber. In one embodiment the method can also include the steps ofgenerating a plasma arc in the plasma chamber using an arc current above50 amps, and controlling a shield gas composition of a shield gas flowsuch that when the hole is being cut the shield gas compositioncomprises an amount of nitrogen such that any potential bevel of theside wall of the hole is substantially eliminated.

The invention features, in one aspect, a plasma torch system for cuttinga hole and contour in a workpiece. In one embodiment the plasma torchsystem includes a plasma torch tip configuration including a nozzle andan electrode that defines a plasma chamber, a plasma arc is generated inthe plasma chamber. In one embodiment the plasma torch system alsoincludes a shield gas supply line for providing a shield gas flow to theplasma torch tip and a control unit for controlling a composition of theshield gas flow. In one embodiment the control unit controls thecomposition of the shield gas flow such that while cutting the contourthe shield gas flow comprises a first shield gas composition and whilecutting the hole the shield gas flow comprises a second shield gascomposition. In one embodiment the improvement comprises a computerreadable product tangibly embodied in an information carrier, operableon the control unit, the computer readable product containing cuttinginformation for the plasma arc torch system including instructions thatselect the first shield gas composition when cutting the contour andselect the second shield gas composition when cutting the hole.

In another aspect, the invention features a component that includes acomputer readable product tangibly embodied in an information carrier,operable on a CNC for use in a plasma torch system. In one embodimentthe computer readable product includes cutting information for cutting ahole and a contour from a workpiece using a plasma arc torch, includinginstructions such that while cutting the hole a shield gas flowcomprises a second shield gas composition and when the contour is cutthe shield gas flow comprises a first shield gas composition.

In still another aspect, the invention features a computer numericalcontroller for controlling cutting parameters of a plasma torchincluding a composition of a shield gas flow. In one embodiment thecontroller includes a processor, an electronic storage device, aninterface for providing control instructions to a plasma arc torch, anda look up table for selecting the composition of the shield gas flow forthe plasma torch. In one embodiment the controller controls thecomposition of the shield gas flow according to whether the plasma torchwill cut a hole or a contour in a workpiece.

Any of the aspects above can include one or more of the followingfeatures. The second shield gas composition can comprise less nitrogenthan the first shield gas composition such that a bevel of an edge ofthe hole is substantially eliminated. In some embodiments the workpieceis mild steel, in some embodiments the first shield gas composition isair, and in some embodiments the second shield gas composition isoxygen. The second shield gas composition can also consist essentiallyof oxygen during hole cutting. In one embodiment a flow rate of theshield gas flow is reduced during hole cutting. A cutting speed of thetorch can be reduced during hole cutting. In one embodiment, controllingthe cutting parameters can further comprise controlling the secondshield gas composition according to a ratio of a diameter of the hole toa thickness of the workpiece. The ratio can be less than or equal to2.5. In some embodiments the ratio is less than or equal to 1. In someembodiments the ratio is less than or equal to 0.7 and/or limited by thesize of the pierce penetration.

Any of the aspects above can also include one or more of the followingfeatures. In one embodiment controlling the cutting parameters caninclude current ramping sequences for arc termination where the rampingis constant for both contour cutting and hole cutting. The step ofcontrolling the cutting parameters can also further comprise controllingan amount of nitrogen in the shield gas flow such that the second shieldgas composition contained less nitrogen as a percentage of the totalvolume than the first shield gas composition whereby a bevel of an edgeof the hole is substantially reduced.

Any of the aspects above can also include one or more of the followingfeatures. In one embodiment a method can include the step of providing acomputer readable product tangibly embodied in an information carrier,operable on a CNC for use with a plasma torch system, the computerreadable product containing cutting information for the plasma arc torchincluding instructions that select the first shield gas composition whencutting the contour and select the second shield gas composition whencutting the hole. In some embodiments the second shield gas compositioncan be selected according to a ratio of the diameter of the hole to thethickness of a workpiece. And in some embodiments the cuttinginformation include instructions such that when a hole is cut thecontrol unit controls the second shield gas composition according to aratio of a diameter of the hole to a thickness of the workpiece.

One advantage of the present invention is that it produces high qualityplasma cut holes, while maintaining productivity and dross levelstypically achieved on contour cuts. Another advantage of the presentinvention is that it also minimizes impact on overall part cost bylimiting the use of more expensive shield gas mixtures to short durationhole cuts.

A further advantage of the present invention is that it improves timeefficiency by allowing the operator to use a single configuration oftorch consumables when cutting holes and contours in a single workpiece,while simultaneously preventing the quality deterioration seen whenusing the prior cutting technique of a single shield gas for both holeand contour cutting.

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent from the following descriptionand from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features will be more fully understood by reference tothe following detailed description when taken in conjunction with theaccompanying drawings, which are illustrative and not necessarily toscale.

FIG. 1 is a diagram of a known mechanized plasma arc torch system,showing a torch mounted on a table.

FIG. 2 is a cross sectional view of a known plasma arc torch tip.

FIG. 3 is a sample workpiece showing anticipated hole and contour cutoutlines.

FIG. 4 is a block diagram of a plasma arc torch system with a proposedgas system.

FIG. 5 is a flow diagram that shows how gas flows can be manipulatedaccording to an embodiment of the invention.

FIG. 6 depicts a movement path during a hole cutting operation.

FIG. 7 is a table illustrating different gas combinations that can beused with an embodiment of the invention for cutting mild steel.

FIG. 8A is an illustration of tolerance measurements used to determinecylindricity of a hole.

FIG. 8B is a cross section of a hole cut with the prior art cuttingprocess.

FIG. 9 is a cross section of a hole cut with an embodiment of thepresent invention.

DETAILED DESCRIPTION

In the present invention, a first shield gas composition is used whencutting the contour, and a second shield gas composition is used whencutting one or more holes or small internal feature in a singleworkpiece while using a single plasma torch consumable configuration.

As used herein, a hole is a shape having a diameter (or dimension) toworkpiece (plate) thickness ratio of approximately 2.5 or smaller.Referring to FIG. 3, by way of example, shows a 6×6 inch square piece of0.5 inch thick plate steel 100 that in one embodiment, could be cut froma larger workpiece (not shown). A 1 inch diameter hole 105 in the 0.5inch thick plate of steel 100 would have a ratio of 2. A hole, as usedherein, can be categorized as a small internal part features that arenot necessarily round, but where a majority of the features havedimension that are about 2.5 times or less than the thickness of thematerials, for example a 1 inch square 110 in the V2 inch plate steel100. All other features, are referred to herein as contours which caninclude both straight 115 or curved 120 cuts.

A torch system configuration that can be used with one embodiment isshown in FIG. 4. In one embodiment, the shield gas composition forcutting a hole is O₂. In some embodiments, the shield gas compositionselected when cutting a hole contains less nitrogen than the shield gascomposition used when cutting the contour. In some embodiments, theshield gas composition used when cutting a hole can include He, N₂, O₂,or combinations thereof.

FIG. 4 is a block diagram of a plasma arc torch system including anautomatic gas control system according to an embodiment of theinvention. The plasma torch system can include all of the elementsdescribed above in connection with FIG. 1. Additionally, the torchsystem can include a gas console 40 that provides plasma and shield gasto the plasma arc torch 41. The plasma gas and the shield gas flows fromthe gas console 40 through gas supply lines 42 to, in some embodiments,a gas selection consol 45 and a gas metering consol 44 allows for themixing of different types of gases, before the gas mixture continues tothe plasma torch 41. The gas selection consol 45 allows the selectionand mixing of one of a plurality gases, the selected gases can then bemetered by the gas metering console 44. The gas consol can receive gasinputs including: oxygen, nitrogen, F5, H35, H5, and air. The gasmetering consol 44 can then measure the plasma gas and shield gas. Thiscontrol configuration allows for the plasma system to rapidly change therequired shield gas or gas mixture for hole piercing, hole cutting, orcontour cutting. For example, when cutting a hole, in one embodiment ofthe present invention, the gas consol 40 provides air as the shield gasduring the piercing process and when the piercing of the metal plate iscomplete, the gas consol 40 automatically switches the shield gas to O₂for hole cutting. When the plasma system moves to cut a contour, the gasconsol 40 can switch the shield gas back to air as the shield gas forboth the piercing and cutting processes. Such rapid switching can bedirected by code or programming in the CNC 12.

The gas supply line 42 that carry the shield gas flow are referred to asshield gas supply line 42A in some embodiments. And in some embodiments,the gas supply line that carry plasma gas flow are referred to as plasmagas supply line 42B. In some embodiments, the composition of the plasmagas flow is controlled using valves 47. In some embodiments the valves47 are on-off solenoid valves, and in some embodiments the valves arevariable solenoid valves. In some embodiments, the plasma and shield gascan be O₂, Air, He, N₂ or some combination thereof. The gas meteringconsole 44 can also include a venting valve 48 which can also be anon/off valve or a solenoid valve. In some embodiments, the vent valve 48is used to enable rapid switching of the plasma gas and shield gas.

The CNC 12 can be any computer that controls a plasma torch system. ACNC 12 can have a processor, electronic storage device, and an interfacefor providing control instructions to a plasma arc torch. The storagedevice can be internal or external and can contain data relating to thepart to be cut in the workpiece. In other embodiments, the CNC 12 can bemanually programmed, and in some embodiments the CNC 12 can include acomputer readable product that includes computer readable instructionsthat can select or configure operating parameters of the plasma torchsystem.

An example of computer readable instructions is below. The instructionscorrespond to a round hole cut into a square contour cut using aHypertherm Automation Voyager CNC controller with an HPR 260 AutogasConsole, all manufactured by Hypertherm, Inc. of Hanover, N.H. In theexemplary code below used with the Hypertherm Automation CNCcontrollers, the code provides two separate cut charts for the hole (G59V503 F1.01 through G59 V507 F31) and for the contour (G59 V503 F1through G59 V507 F31). In some embodiments, other forms of code, orcomputer readable instructions can be used with one or more cut chartsto provide a similar, or even identical final output. Notably, the leftcolumn contains the referenced code lines; the right column providesgeneral a generic explanation of the instructions contained in each codeline.

G20 English units are sets G91 Incremental programming mode G59 V503F1.01 Load a custom cut chart for a hole G59 V504 F130 ″ G59 V505 F3 ″G59 V507 F31 ″ G00X1.7500Y-1.7500 Move to hole center M07 Plasma startG03X-0.0970I-0.0485 Hole motion G03X0.0015Y0.0168I0.0970 ″G03X0.0212Y-0.0792I0.0955J- ″ 0.0168 M08 Plasma stop G59 V503 F1 Loadcut chart for contour cut G59 V504 F130 ″ G59 V505 F2 ″ G59 V507 F31 ″G00X-1.6757Y1.5624 Move to contour start location M07 Plasma startG01X0.2500 Contour motion G01X3.0000 ″ G01Y-3.0000 ″ G01X-3.0000 ″G01Y3.0000 ″ G01Y0.2500 ″ M08 Plasma stop M02 End of program

In some embodiments the computer readable products are referred to ascut charts. In some embodiments, the computer readable product (notshown), or cut charts, contains cutting information includinginstructions that select a first shield gas when the torch 41 is cuttinga contour in a workpiece and select a second shield gas composition whenthe torch is cutting a hole in the same workpiece. In some embodiments,the cut chart contains information that selects the shield gascomposition based on the type of cut, that is a contour cut or a holecut. In some embodiments, the CNC is able to rapidly switch from oneshield gas to another depending on the instructions contained in the cutchart. In some embodiments, the torch operator selects the shield gascomposition and the CNC 12 only provides signals to control, forexample, the plasma gas supply line valves 44 based on the informationinput from the torch operator.

In some embodiments, the torch operator selects a cutting program thatincludes both hole and contour cutting instructions. And in someembodiments an operator selects a hole cut chart and a contour cut chartthat are designed to execute consecutively. In some embodiments, thehole cut will be positioned within the contour cut anticipated by theCNC 12 on the workpiece. When a cutting program includes instructionsfor both hole cuts and contour cuts, the cut chart will include furtherinstructions such that the hole is cut first using a second shield gascomposition and then the contour cut is cut using a first shield gascomposition. Cutting the holes first within a profile of the anticipatedcontour cut prevents movement of the workpiece while the holes are beingcut, thus eliminating deviations that would occur if the contour cuts ofthe part were cut first and the holes cut second.

In other embodiments, the computer readable product is nesting software,such as is made by MTC of Lockport, N.Y. Nesting software can providecode that designates when the first shield and second shield gases areto be used based upon CAD drawings of the part to be cut. The nestingsoftware can use the CAD drawing to identify the holes or small internalfeatures based upon the ratio of hole diameter to the thickness of theworkpiece. The nesting software can then provide instructions to the CNC12 so that the first shield gas is used when cutting contours and thesecond shield gas is used when cutting holes. Alternatively, the CNC caninclude software that selects the appropriate shield gas, for holecutting and contour cutting without being provided instructions from thenesting software.

FIG. 5 is a flow chart depicting how a processor, such as a computerizednumeric controller (CNC), can be used to manipulate gas flows toimplement principles of the invention. FIG. 5 shows an embodiment of theflow operations that can be contained within a computer readable productwhich is embodied in an information carrier. Other embodiments are alsowithin the scope of the invention. As shown in FIG. 5, a CAD filecontaining the part to be cut is provided to the CNC 510, or nestingsoftware, and based on instructions contained in the cut chart the CNCselects the shield gas composition. In another embodiment, instructionscontained in the nesting software determine the shield gas composition.In some embodiment, once the CNC uses the computer readable instructionsto determine if a hole or contour is being cut, the torch is powered on520 and the arc is transferred to the workpiece 530. When the arc isinitiated, the initiation shield and plasma gas is used, for example thecombinations shown in FIG. 7. After the arc is transferred to theworkpiece, the torch is lowered to the workpiece and the arc pierces theworkpiece 530. In one embodiment, the arc pierces the workpiece usingair as the pierce shield gas. Once the pierce step is completed, the CNCuses the computer readable instructions to select the appropriate shieldgas depending on whether a hole or a contour is to be cut. In someembodiments, the determination as to whether a hole or a contour is tobe cut (and selection of the appropriate shield gas composition) isbased on an examination of the dimensions of the hole in relation to thethickness of the workpiece. In one embodiment, if the diameter of thehole is about 2.5 times or less than the thickness of the workpiece,then a hole is to be cut, and the CNC selects the second shield gas 550.In some embodiments the shield gas composition selected for hole cuttingis O₂; and in some embodiments the shield gas composition is O₂, He, N₂,or a combination thereof. In some embodiments, the instructionsregarding the shield gas compositions are included in the instructionson the cut chart. Once the second shield gas is selected, the CNC willcontrol the shield gas flow such that the second shield gas compositionflows through the shield gas supply lines. The hole is then cut 560 inthe workpiece using the second shield gas composition as determined bythe instructions contained in the cut chart, or designated by thenesting software. After one or more holes are cut in the workpiece, theCNC initiates the contour cutting operations 570. When the CNC initiatesthe contour cutting operation, the arc is again initiated 530 using theinitiation shield and plasma gas for contour cutting. The arc thenpierces the workpiece 540, and as the contour cutting begins, the CNCselects the first shield gas for the contour cutting operation 580.

If it is determined that a contour is being cut, then the CNC selectsthe first shield gas composition for the contour cut 580. Theidentification of a contour can be selected based on the shape of thecut or in the case of an internal feature, it may be based on a ratio ofthe diameter of the opening to be cut to the thickness the workpiece. Insome embodiments when cutting a contour the arc initiation, the piercingof the workpiece, and the contour cut are all performed using a singleshield gas composition, that is, the first shield gas composition. Insome embodiments, the shield gas during the arc initiation and thepiercing of the workpiece is different than the shield gas used whencutting the contour shape in the workpiece.

When cutting a hole or contour in a workpiece, the same operationalsteps can be followed, although different shield gas compositions may beselected for each step. FIG. 6 shows an exemplary movement path followedduring hole cutting, the movement path is traced out along the top of aworkpiece. First, the plasma gas and shield gas flow are initiated,along with the arc current. The initiation of the gas flows and thecurrent arc can vary depending on the consumable and torch configurationbeing used by the operator. U.S. Pat. No. 5,070,227, U.S. Pat. Nos.5,166,494, and 5,170,033, all assigned to Hypertherm®, Inc. andincorporated herein by reference in their entireties, describe variousgas flow and current settings that can be used during initiation,operation, and shut-down of the plasma arc, and cutting process. Afterthe plasma arc is initiated, it is transferred to the workpiece. Oncethe arc is transferred to the workpiece, in some embodiments, the torchheight is lowered using the torch height controller. A hole cut is begunin a workpiece by first piercing the workpiece using the plasma arc.Once the workpiece is pierced through by the plasma arc, the shield gasis switched to a second shield gas the composition of which can beoptimized for hole cutting. In some embodiments the torch will begin totranslate across the workpiece to cut the hole into the workpiece alongthe hole cut pattern which can be, in some embodiments, determined bythe part drawing plasma and shield. FIG. 6 shows an exemplary embodimentincluding the piercing position 51, the circling start position 52, thecircling end position 53, and the edge 54 of the hole cut in theworkpiece.

FIG. 7 is a table illustrating examples of gas combinations that can beused with an embodiment of the invention. In one embodiment, the gasesare selected to provide optimal gas cutting properties based upon theplasma torch operation, such as hole or contour cutting to be performed.The gases being shown in this figure are for mild steel cuttingapplications. Though the present invention can be used in cutting othermaterials, different shield gases can be better suited for suchmaterials. In some embodiments, a mixture of He and N₂ can be used inplace of oxygen for the hole shield gas cutting stainless steel oraluminum.

In the embodiment demonstrated in FIG. 7, while cutting either a contouror a hole, the system provides air as the plasma gas and the shield gasduring plasma arc initiation. Air is used as the plasma gas because ittends to provide better consumable life compared to O₂ during arcinitiation. Once the arc is initiated and transferred to the workpiece,the plasma gas is changed to O₂ and the shield gas remains as air forthe piercing process. In this instance, the plasma gas is switched tothe gas that is appropriate for the nozzle design, in this embodimentO₂, in order to prevent damage to the nozzle as the current is ramped upto the cutting current. In most cases it is desirable that the cuttinggas be present at the time full cutting current is reached. The shieldgas for the piercing process, on the other hand, remains as air. Airshield gas for piercing operations has been shown to leave a smallerpierce penetration which limits waste in the workpiece. Once theworkpiece is pierced, the plasma torch will begin cutting along the edgeof the penetration with the motion of the torch. It is important todistinguish between piercing and cutting. In piercing, the torch isgenerally stationary and the object is to make a penetration completelythrough the workpiece. Cutting, on the other hand, involved moving thetorch by severing exposed edges to create the desired shape.

Referring again to the table of FIG. 7, after the piercing step, theshield gas can be selected based upon the type of cut: a contour or ahole. In cutting a contour, the shield and plasma gas remain unchanged.The combination of O₂ plasma gas and air shield gas allows straightdross free edges and fast cut speeds when cutting contours using an O₂plasma gas and air shield gas combination, however, tends to create ahole with a large degree of taper or bevel, creating a poor qualityhole. By keeping O₂ as the plasma gas and switching the shield gas alsoto O₂ when cutting holes or small internal features, the taper of thehole can be reduced if not eliminated. Taper is reduced by using an O₂shield gas when cutting mild steel compared to air because the amount ofnitrogen in the shield gas is reduced. Thus, other gases or gascompositions with low nitrogen content could be used in the embodimentin FIG. 7. In other embodiments, a shield gas with different compositioncombinations can be used when cutting holes.

As mentioned previously, the shield gas composition has been found toaffect the taper, or bevel, of the edge of a hole cut that is beingperformed. The bevel can be measured by the cylindricity of thecompleted hole cut. Cylindricity is defined as a tolerance zone that isestablished by two concentric cylinders between which the surface of acylindrical hole must lie as illustrated in FIG. 8A. In FIG. 8A thetolerance zone can be defined as the space between the two arrows 81.The smaller the tolerance zone, the more the surface represents aperfect the cylinder. A large taper or bevel in a hole, on the otherhand, will result in a large tolerance zone. Cylindricity of a hole canalso be measured using a coordinate-measuring machine (“CMM”).

FIG. 8B is another example of a cross sections of a hole cut using priorart cutting processes, that is, using the same shield gas compositionfor both a contour cut and a hole cut in the same workpiece. In FIG. 8Bthe cylindricity (“taper” or “bevel”) of the hole can be measured byforming concentric cylinders with a diameter equal to the diametermeasurement at the top 71, middle 72, and bottom 73 of the edge 74 ofthe hole. The greatest difference between the diameters is illustratedby the space between the arrows 81. The large difference between theradiuses of the two datum cylinders in FIG. 8B indicates a poor qualityhole. Such holes can require significant post cutting treatment.

FIG. 9 is a cross section of a hole cut with an embodiment of thepresent invention. In FIG. 9 the cylindricity (“taper” or “bevel”) ofthe hole can be measured also by forming concentric cylinders with adiameter equal to the diameter measurement at the top 71, middle 72, andbottom 73 of the edge 74 of the hole. In FIG. 9, it can be seen that thebevel or taper of the edge of the hole cut is significantly reduced ascompared to the bevel of the hole edge in FIG. 8A and FIG. 8B. Further,the reduced cylindricity can also be seen by the reduced distancebetween the arrows 81 as compared to FIGS. 8A and 8B. With the reducedbevel or taper of the edges of the hole, the cylindricity tolerance zonebetween the two concentric cylinders is minimal and resulting in a muchhigher quality hole, requiring no post cutting treatment.

While the invention has been particularly shown and described withreference to specific embodiments, other aspects of what is describedherein can be implemented in cutting systems of ordinary skill in theart. It should be understood by those skilled in the art that variouschanges in form and detail can be made without departing from the spiritand scope of the invention as defined by the appended claims.

1. A method for cutting a hole and a contour in a workpiece, the methodcomprising: providing a plasma arc torch having a nozzle and anelectrode that define a plasma chamber in which a plasma arc isgenerated, a shield gas supply source for providing a shield gas flow tothe plasma arc torch, and a control unit for controlling cuttingparameters including shield gas composition and a cutting speed; cuttingthe hole with the plasma arc torch while using the control unit tocontrol the cutting parameters for a second shield gas composition and asecond cutting speed changing the shield gas composition from the secondshield gas composition to a first shield gas composition, the secondshield gas composition containing less nitrogen as a percentage of atotal shield gas volume than the first shield gas composition; andcutting the contour with the plasma arc torch at a first cutting speedwithout changing the nozzle and electrode by using the control unit tocontrol the first shield gas composition and the first cutting speed,the second cutting speed being slower than the first cutting speed. 2.The method of claim 1 wherein controlling the cutting parameters furthercomprises controlling the second shield gas composition according to aratio of a diameter of the hole to a thickness of the workpiece.
 3. Themethod of claim 2 wherein the ratio is less than or equal to 2.5.
 4. Themethod of claim 1 further comprising: providing a non-transitorycomputer readable product tangibly embodied in an information carrier,operable on a CNC for use with a plasma torch system, the computerreadable product containing cutting information for the plasma arc torchincluding instructions that select the first shield gas composition whencutting the contour and select the second shield gas composition whencutting the hole.
 5. The method of claim 4 further comprising selectingthe second shield gas composition according to a ratio of a diameter ofthe hole to a thickness of a workpiece.
 6. A plasma torch system forcutting a hole and contour in a workpiece, the plasma torch systemcomprising: a plasma torch tip configuration including a nozzle and anelectrode, the nozzle and electrode defining a plasma chamber in which aplasma arc is generated; a shield gas supply source for providing ashield gas flow to the plasma torch tip; and a control unit forcontrolling a composition of the shield gas flow, the improvementcomprising: a non-transitory computer readable product tangibly embodiedin an information carrier, operable on the control unit, the computerreadable product containing cutting information for the plasma arc torchsystem including instructions that select a first shield gas compositionand a first cutting speed when cutting the contour and select a secondshield gas composition and a second cutting speed when cutting the hole,wherein the second shield gas composition contains less nitrogen as apercentage of a total shield as volume than the first shield gascomposition and the second cutting speed is slower than the firstcutting speed.
 7. The plasma torch system of claim 6 wherein secondshield gas composition comprises less nitrogen than the first shield gascomposition such that a bevel of an edge of the hole is substantiallyeliminated.
 8. The plasma torch system of claim 6 wherein the workpieceis mild steel, the first shield gas composition is air, and the secondshield gas composition is oxygen.
 9. The plasma torch system of claim 6wherein the second shield gas composition consists essentially of oxygenduring hole cutting.
 10. The plasma torch system of claim 6 whereinsecond shield gas is composed of one or more of helium, oxygen, andnitrogen.
 11. The plasma torch system of claim 6 wherein the firstshield gas composition is air.
 12. A component comprising: anon-transitory computer readable product tangibly embodied in aninformation carrier, operable on a CNC for use in a plasma torch system,the computer readable product comprising cutting information for cuttinga hole and a contour from a workpiece using a plasma arc torch, suchinformation including instructions such that while cutting the hole, ashield gas flow comprises a second shield gas composition and a cuttingspeed comprises a second cutting speed, and when the contour is cut, theshield gas flow comprises a first shield gas composition and the cuttingspeed comprises a first cutting speed, the second shield gas compositioncomprising less nitrogen as a percentage of a total shield gas volumethan the first shield gas and the second cutting speed being slower thanthe first cutting speed.
 13. The component of claim 12, the cuttinginformation further comprising instructions such that when a hole is cutthe control unit controls the second shield gas composition according toa ratio of a diameter of the hole to the thickness of the workpiece.